SoftBank president Masayoshi Son announces the company’s net profit at a Tokyo press conference in October 2013.

Masayoshi Son, the fiery head of SoftBank Corp., has never been one to shy away from regulatory battles.

That’s useful to recall as Mr. Son ponders a possible bid by Sprint Corp. — the No. 3 U.S. wireless carrier, which SoftBank acquired this year — for the No. 4-ranked T-Mobile U.S. Inc., a move that experts say could hit regulatory headwinds.

As recently as this summer, Mr. Son, whose SoftBank also owns Japan’s No. 3 wireless carrier, accused Japanese telecommunications regulators of ineptitude and threatened to sue, after a news report said they had decided to allocate bigger chunks of the 2.5-gigahertz wireless spectrum to rival KDDI Corp.

Regulators denied a decision had been made yet, and Mr. Son himself later reflected on the incident with a laugh, telling news reporters it was “the first time in years that I got so angry…. I thought I had grown up.”

In the end, the bulk of the spectrum did go to KDDI. Mr. Son said subsequently that while the company may eye legal measures, it would tread carefully lest a legal spat with regulators hurt its business in other ways.

“It’s easy to sue. We’ve done it before,” Mr. Son said in a press conference at the end of July. “But when we did so, we were unable to gain any access to regulators. I would try to make appointments and not be able to meet anyone for years. It caused much damage to our operations. If I sue this time, I expect repercussions.”

A ministry official at the time denied the regulator would retaliate. SoftBank has not filed a suit.

That’s only one of the most recent regulatory tussles for Mr. Son — a billionaire whose ventures have jumped from software distribution to Internet-services investment to broadband to mobile phones. Those confrontations have had mixed results.

SoftBank actually did sue the telecommunications ministry in 2005 over a decision to allocate frequencies in the 800-megahertz band to NTT Docomo Inc. and KDDI — Japan’s No. 1 and 2 ranked cellphone companies — while rejecting SoftBank.

SoftBank later withdrew its suit after the company applied for and received a license to operate within the 1.7-gigahertz band.

That time, the official pushed NTT to cooperate, and Mr. Son’s high-speed Internet network became Japan’s fastest, cheapest and most popular, sparking a broadband revolution in the country.

In 1996, after SoftBank had bought a 21.4% stake in Japanese broadcaster Asahi National Broadcasting Co. with News Corp., the two had a run-in with Japanese telecom regulators, which rejected their plan to put an Australian on the board of the Japanese company.

The telecom ministry cited a law that banned granting a license to broadcasters with foreign nationals as directors. News Corp. said it would be willing to put a Japanese national on the board instead, but a year later, the two companies sold their jointly-held stake to Asahi Newspaper Co., the publisher of the Asahi Shimbun paper.

Still, those battles were all with Japanese bureaucrats, whom Mr. Son has at various times accused of being ignorant and out of touch with global standards. He’s generally been complimentary about U.S. regulators, calling them a model of transparency in contrast.

“In America, these important decisions are debated in the open, so that everyone can see the numbers and the facts,” Mr. Son said in July, about his feud with Japan’s regulators over their spectrum-allocation decision. “It is ridiculous that a decision as important as the allocation of bandwidth — assets that belong to the people — should be made behind closed doors.”

SoftBank is now beefing up its lobbying staff in Washington, D.C., according to a person familiar with the matter, in order to fortify itself for any regulatory scraps ahead.

About Japan Real Time

Japan Real Time is a newsy, concise guide to what works, what doesn’t and why in the one-time poster child for Asian development, as it struggles to keep pace with faster-growing neighbors while competing with Europe for Michelin-rated restaurants. Drawing on the expertise of The Wall Street Journal and Dow Jones Newswires, the site provides an inside track on business, politics and lifestyle in Japan as it comes to terms with being overtaken by China as the world’s second-biggest economy. You can contact the editors at japanrealtime@wsj.com